1. Field of the Invention
The present invention relates to an overhead cable (including an overhead power transmission line and an overhead ground wire) which prevents wind noise and corona noise and, at the same time, reduces the wind pressure.
2. Description of the Related Art
When wind blows against a laid overhead power transmission line, for example, a Karman vortex is generated on the downwind side of the overhead power transmission line and wind noise is caused. To prevent this wind noise, it is effective to provide a projection at the outer periphery of the overhead power transmission line to disturb the Karman vortex.
For this reason, conventionally, for example as shown in Japanese Examined Patent Publication (Kokoku) No. 53-14146, a spiral rod made of metal, formed by an aluminum wire etc., was wound around the outermost layer of the overhead power transmission line to provide the projection and this projection was used to disturb the Karman vortex and thereby prevent the wind noise.
When winding a spiral rod made of metal around the outermost layer of the overhead power transmission line, however, there arises a problem that corona noise is apt to be generated during rain.
The cause of this is that the electric field becomes stronger at the surface of the projection, so when rain is deposited there to form drops of water which subsequently drop down, a strong discharge occurs.
To prevent the corona discharge described above, it is known to form the spiral rod by a semiconductor material and an insulating material (refer to Japanese Unexamined Patent Publication (Kokai) No. 3-277114). When such a spiral rod is used, since the insulating material does not have any effect on the electric field distribution, the rod is effective for suppressing corona noise at the time of rain.
However, the work of winding the spiral rod around the outermost layer of the overhead power transmission line was very cumbersome.
Therefore, an overhead power transmission line formed by stranding an outwardly projecting strand at the outermost layer of the overhead power transmission line has been proposed (Japanese Examined Patent Publication (Kokoku) No. 6-42328).
When preliminarily winding the outwardly projecting metal strand around the outermost layer of the overhead power transmission line in this way, there is the advantage that it becomes unnecessary to wind a spiral rod later.
When stranding an outwardly projecting metal strand at the outermost layer of the overhead power transmission line, however, the weight of the overhead power transmission line is increased, and therefore there is a problem that the strength of the cable supporting structures such as the steel towers and insulators has to be increased. Further, there is a problem that it is necessary to newly prepare the gripping portions of spacers, dampers, and other parts for gripping the overhead power transmission line to match with the outwardly projecting metal strand.